fix: wrap top-level array params in anyOf to also accept JSON-encoded strings

[cheafon]

Problem

When calling notion-create-pages from Claude Code (or Cursor), the pages
array parameter arrives as a JSON-encoded string instead of an actual array,
causing validation to fail:

Invalid input: expected array, received string
Too big: expected string to have <=100 characters

This is a known MCP client behavior where top-level array parameters get
double-serialized (JSON.stringify'd) before being sent.

Root Cause

withStringFallback() in parser.ts already handles this for object
parameters by wrapping them in anyOf: [schema, { type: 'string' }] —
allowing both the native type and a JSON-encoded string. However, this
treatment was missing for array parameters.

The runtime fix (deserializeParams() in proxy.ts) already correctly
converts "[...]" strings back to arrays — so only the schema validation
needed to be updated.

Fix

Apply the same anyOf: [array, string] wrapping to array-typed parameters
in withStringFallback(). This is consistent with the existing object
handling and resolves the schema validation error.

Testing

• Added a test case in parser.test.ts covering an array request body
  parameter (mirrors the notion-create-pages / pages scenario)
• Updated existing parser-multipart.test.ts expectations to reflect the
  new anyOf structure for array properties
• All 75 tests pass

Fixes #176